The invention relates to a device for the re-condensation of low-boiling gases evaporating from a liquid gas container by means of a cryo-generator. With such a device for example a superconductive magnet which is cooled by immersion into liquid helium can be operated over an extended period by re-condensation of the helium evaporated. The device is a small refrigeration apparatus, a so-called cryo-cooler. In a similar way, such a device is used in connection with a superconductive magnet of high-temperature superconductive material which is cooled by immersion into liquid nitrogen.
Below the present state of the art is described shortly (see also FIG. 4):
The cryo-container 1 consists of an inner container 2, which is filled with the low-boiling liquid gas, for example, liquid helium, up to a level 7. The superconductive apparatus, typically a magnetic coil 5 including the power supply lines 6a, 6b is immersed into the liquid gas. The helium evaporating as a result of the heat supplied to the container 2 is conducted, by way of a narrow tube 8, to the ambient or rather to a collecting container. For reducing the heat influx, the helium container 2 is surrounded by an enclosure 3 and the space between the inner container 2 and the outer enclosure 3 is evacuated. For further reducing the heat influx, a radiation shield 4 is arranged in the vacuum space between the container 2 and the enclosure 3. The radiation shield 4 is cooled by the helium gas by way of a contact ring 10 disposed on the tube 8. On one hand, the tube 8 should be as narrow as possible in order to reduce the heat influx but, on the other hand, if, accidentally, the magnet becomes suddenly normally conductive, the tube 8 should have a sufficiently large cross-section to permit the discharge of the additional gas generated in order avoid in that case an excessive pressure increase in the container 2.
When the helium level has dropped below a certain height the helium must be replenished from a transport container. This requires substantial efforts and expenditures.
There are small cooling devices (cryo-generators) by which the helium evaporating from the helium bath can again be liquefied and returned directly to the cryo-container. Some of these devices have two- or several stages and provide sufficient cooling energy for the cooling of radiation shields. The most important embodiments of such cryo-generators are presently the pulse tube cooler and the Gifford-McMahon cooler.
As far as this is possible with such low temperature cooling apparatus such a cryo-generator should be easy to handle, uncomplicated in its operation and easy to service. The low temperature-boiling gases used in these cooling apparatus are helium, He, Hydrogen H2; Neon, Ne; nitrogen, N2 which are also used in the superconductor technology as coolants.
A cryo-generator consists basically of cooling equipment with a so-called cold head. This cold head is mounted outside onto the apparatus and extends into the tube 8 down to the container 3 for the liquid gas. There, the cold area 26 is exposed to the liquid level 7 of the liquid gas. The single-stage cooling apparatus is so designed and installed that it can be removed and re-installed without heating the liquid gas. The cold head comprises a regenerator 21 and a pulse tube 23 with a heat exchanger 25 disposed therebetween. The heat exchanger 25 is embedded in the cold area 26, which is exposed toward the liquid gas bath.
The components regenerator 21, pulse tube 23 are surrounded each by a thermally isolating enclosure/heat shield (20, 30, 31, 32) in order to prevent thermal coupling to the outside or at least to maintain it within acceptable limits.
The cooling apparatus that is the cold head may be of different design, but it includes generally at least two stages. It also extends into the tubular neck 8 and its last stage cold area 28 is disposed above the liquid gas bath. Also, such a multistage cold head can be removed and re-installed without heating the liquid gas bath. Each stage of the cold head consists of a regenerator 21, and, respectively, 22 and a pulse tube 23 and, respectively, 24, with a heat exchanger 25 and, respectively, 27 disposed therebetween. Each heat exchanger is contained in a cold area 26 or, respectively, 28. The cold area of the last stage extends with its exposed surface into the cold vapor space of the liquid gas container 2. The components, the regenerator 21 and respectively, 22, the pulse tube 23 and respectively, 24 of the respective stage are, like in the single stage embodiment, each surrounded by a thermally insulating tube 29, 30, 31, 32. All the cold areas 26, except for the last one, are disposed in the direction toward the next following stage co-axially opposite a heat transfer ring 10, which is disposed at the respective location in the tubular neck 8 in good heat transfer relationship. The respective cold head area 26 extends in an axially movable manner, with a small equidistant gap around the circumference, into the associated heat transfer ring 10, without coming into contact therewith at any point. In this way, there is always a gas passage open from the vapor space above the liquid gas bath to the flange of the cold head. The multistage cooling apparatus extending into the tubular neck 8, which is mounted onto the flange cover 33 that is bolted onto a connector flange 9 of the corner wall 3, can expand axially as a result of thermal effects without restrictions.
It is the object of the present invention to provide an improved device for the re-condensation of low boiling gases evaporating in a liquid gas container.